The present invention relates to a method and apparatus for applying a laser beam to a working surface. The invention is particularly useful in a surgical laser for the removal of undesired tissue by ablation, and is therefore described below with respect to this application.
When removing undesired tissue by ablation with the use of a laser beam, the follow criteria should be maintained: (1) minimal thermal damage to the surrounding tissue; (2) avoidance of char residue over the ablated area; and (3) uniform ablation of the tissue without causing holes or cracks in the surrounding tissue.
To fulfill criteria (1) and (2), a laser power density of 40 W/mm.sup.2 on tissue, and a lasing duration of one millisecond, should be maintained so that the energy density will reach a level of 40 MJ/mm.sup.2. Under these conditions, the char residue heats up to a temperature of over 500.degree. , whereupon it is oxidized and converted to gas. The high power density ablates tissue at such a rapid rate, that thermal damage does not permeate a depth of more than 100 .mu.m, therefore achieving minimal thermal damage.
To fulfill criterion (3), a beam with a diameter of at least 3.5 mm (area of 10 mm.sup.2) should be used. A smaller beam diameter will cause holes and cracks in the tissue.
In order to maintain a power density of 40 W/mm.sup.2 over an area of 10 mm.sup.2, a 400 W laser is required. A medical laser of such power levels is both too large in size and too expensive. Alternatively, a low-powered focused laser beam may be used with the beam scanning an area of 10 mm.sup.2 with relatively high velocity.
It has been previously proposed to displace the laser beam to trace a circular scan over the working surface (e.g., published European Patent Application 0172490), but the scanned area is thus limited to an annular ring. It has also been proposed to scan larger areas by Lisajou patterns, but the Lisajou patterns so formed are such that the velocity of scanning at the center of the area is much smaller (e.g., thirty times smaller) than the velocity on the perimeter. Low scanning velocity in the center causes the beam to linger at the center, thus causing a higher ablation rate in the center than on the perimeter.